Childbirth education will mentally prepare you for labor, and all those great breathing tricks do keep you busy while labor pains are pounding your body. But you don't have to be a rocket scientist to guess that being in physical shape helps, too. Women who work out regularly have insisted on this for years, and now there's research to prove it: A study of 557 pregnant women published in the American Journal of Public Health found that women who engaged in heavy exercise (expending more than 1,000 calories a week) had faster deliveries than those who engaged in only moderate exercise or none at all. (Another plus: Heavier exercise appeared to reduce the risk of premature birth.) Other studies have shown that women who exercised while pregnant gained less weight, lost pounds more quickly after delivery, and required fewer interventions such as the use of forceps or the need for a cesarean section during delivery.

Still, you shouldn't just step outside and start jogging. Exercise during pregnancy must be carefully monitored for your own safety and the health of your baby (talk to your health care provider before beginning this or any other exercise plan). And while overall fitness is always a plus, there are very specific muscle groups that are instrumental in childbirth, and these are the ones on which you'll need to focus. Think of it as squeezing toothpaste out of a tube. The tube is the uterus and your hand represents the abdominal muscles. When squeezing out the toothpaste (the baby), your hand pushes against the tube, and the toothpaste comes out the bottom. In order for the baby to come out during labor, the abdominal muscles must be tight and the pelvic floor muscles must be open and relaxed. Many women in labor make the mistake of simultaneously tightening both sets of muscles. This is like keeping the cap on the toothpaste!

The workout on these pages has been designed to help you isolate and strengthen these key muscle groups. It's ideal if you begin the exercise program in your first trimester -- you'll be in better shape by delivery and are likely to have a more comfortable, energetic pregnancy. But starting even as late as your eighth month -- assuming you have no complications to worry about -- will improve your odds of having an efficient delivery. Just clear it with your doctor, take it slow, and follow the cardinal rule of exercise: If it doesn't feel right, don't do it.

Click ahead for info on your abs muscles and birth

Know thy abdominals

Strong abdominals are a must for pushing. Situps and crunches are the exercises most women associate with abdominal strength, but it's not safe to do them during pregnancy. They put too much stress on your rectus abdominis (the outermost muscle that runs from your breast bone to your pubic bone), and that muscle is the primary support system for your back. The rectus abdominis is divided vertically into two halves which tend to separate slightly during pregnancy anyway -- a common condition caused by your expanded uterus and referred to as diastasis -- and situps or crunches could aggravate this and cause you more back pain. In contrast, the first three exercises here work the all-important transverse abdominis and help to reduce diastasis. All four of the muscles in the abdominal group (the rectus abdominis, the external obliques, internal obliques, and the transverse abdominis) are important, but it's the transverse abdominis that most needs strengthening for labor. This is the muscle we feel when we breathe, sneeze, or cough. It is the deepest abdominal muscle, wrapping around the middle of the abdomen and the back. If you place your hands on your belly -- one above and one below the belly button -- and take a big breath, you can feel your transverse muscle going out and then in, forward and then backward. It constricts and compresses the abdominal cavity like a girdle, and that's exactly what you need to do to push the baby out.

Click ahead for Four exercises that will ease labor

Four exercises to ease labor

The seated transverse * Sit cross-legged on the floor or in a chair with your shoulders lined up with your hips. Place one hand on your belly and the other hand on your back, just above your waist. Take a deep breath, expanding the belly as you inhale ("a belly breath"). Exhale, and as you do so, contract your abs to pull your belly button in, about halfway back to your spine, then all the way to the spine itself. Squeeze and hold, then release to the half way point (don't go all the way back to the expanded position) and repeat again. Each squeeze and release counts as one contraction. * Start with a set of 25 contractions, five times a day. Work up to 100 contractions a set, which should take about two minutes. Continue to perform five sets a day. As you get stronger, start the movement further back toward the spine, making each contraction smaller.

The back-lying pelvic tiltDuring this exercise and the Headlift, don't lie on your back for more than three minutes or you run the risk of compressing your vena cava, a major blood vessel from the heart. If you start to feel dizzy or lightheaded at any point during these exercises, roll onto your side immediately.

* Lie on your back with your knees squared with your shoulders. Place one hand on your belly and the other just under the small of your back. Take a deep belly breath, expanding your abdomen as you do so. Bring your belly button back to your spine, hold, then press your back to the floor as you exhale. Do a set of 10 tilts daily.

The headlift After you've done about four weeks' worth of the Seated Transverse and Back-Lying Pelvic Tilt, you can add Headlifts to your routine.

* Before you begin, wrap a sheet or long scarf (to be used as a "splint") around your middle, pulling the two ends across your belly. Then, lie on your back with your knees squared with your shoulders. Rest your elbows on the floor and hold the ends of the splint together on top of your belly. Bring the belly button back toward the spine, hold it there, then press the small of your back to the floor (essentially, the Pelvic Tilt you've been doing). Then pull the two halves of the splint together as you gently lift your chin to your chest (not too high) and push the belly button further toward the floor. Release the belly button as you lower your head again. Start with one set of 10 headlifts a day. Work up to three sets of 20 headlifts, then three sets of 30. Remember, don't stay longer than three minutes on your back, and if you start to feel dizzy, roll onto your side.

The Kegel Before you were pregnant, you probably gave little thought to the muscles in your pelvic area. Now that's all you hear about, and for good reason: The pubococcygeus (PC) muscle forms a figure eight around the urethra (where you urinate), the vagina, and rectum, enhancing your sex life and helping you hold in your bodily functions until you can get to a bathroom. It's important to keep your PC muscle in shape, but the weight of your pregnant uterus does a number on it. Performing Kegel exercises will help retain the PC muscle's strength and elasticity as well as teach you how to relax it so the baby can better travel through the birth canal.

* Sit with your legs slightly apart (so you don't squeeze your buttocks) and your hands resting on your belly. Close your eyes and pretend you are stopping the flow of urine. Hold that squeeze for 10 seconds, relax and feel it open, and then repeat. Do 20 repetitions five times a day. (If you can't hold the muscle 10 seconds at first, start with 5 seconds, then work up to 8, then 10.)

Julie Tupler, R.N., is a certified fitness instructor and childbirth educator. She is the author of Maternal Fitness (Fireside) and the creator of the Maternal Fitness Prenatal Video series. Stephanie Wood is a mother of two who writes frequently about parenting issues.